Educate me on Piston Weight

[wonky donky]

Jon I have just looked at your spec above I have realised it's very close to to my spec for the HW77/97 except for the piston weight where mine is around the 250gm all in.

What pre-load & spring dimensions are you using? I like to work around 40mm in general; I have a 3 springs I can use depending on shot cycle required but one of which is only suitable for .22.

I'm in the process of having new piston heads CNC machined to take HW seals for the TX, they will screw straight onto my latch rods.

[sd5782]

I am not getting as educated as I would like. In the SCR thread is a nice listing of bore and stroke, ports, SCR, and some piston weights. Maybe some people have some more measured weights for various guns. That would certainly point out the anomalies such as a Mercury at 310 grams.

I have recently gotten a nice little scale, and can measure this stuff. I wish I had it when I was taking some other guns apart. I don't know much now, but I knew less then, and mostly lube tuned, and put in Maccari kits.

I would have to believe an HW30 piston is much lighter than the HW85, and perhaps the 99 is in the middle. I would also guess a proper English sub 12fpe on an HW80 would benefit from a lighter piston than the 15-20fpe guns we have here in the states. A database perhaps might point to 6-8fpe guns being at a certain weight, 8-10 another, etc etc in very general terms. Perhaps from a database one could see that some pleasant shooting guns have X weight pistons, while some real poor shooting models may have Y weight pistons.

Obviously many more factors are involved, but if several manufacturers choose a certain weight while another is way different, it could be enlightening and open up other tinkering attempts. Here is the USA many guns are tuned for max power; 15fpe HW85s, 20fpe HW80s, 15fpe Diana 34s. Some people are discovering the joys of softer tuning some of these guns. Perhaps the piston weight is a factor here, and not just a softer spring.

[DoctorBoudreau]

I'm in the process of having new piston heads CNC machined to take HW seals for the TX, they will screw straight onto my latch rods.

Nice! Keep us posted! Now get those short TP end plugs CNC'd too! :-)

[Leo Bokkum]

Normal cautions apply- it isn't real and like all computer models, crap in = crap out.

Maybe I have something for you to make (a part of) your model a little bit more realistic?
https://www.dropbox.com/sc/z5umfyw4qsfnrst/cGlCX6IjQh

ATB, Leo

[Shed tuner]

Hi Dave, it's more preload than yours to compensate for the light weight piston... I know free spring length is 260mm; when I strip it (may not be until Monday), will confirm preload / wire gauge / coils etc but IIRC it's 3.1 or 3.2mm wire (hard to measure accurately).

Funnily enough I need to email you about what spec springs you may have available...

With this piston weight "issue" I like the cycle with lighter piston, but what I don't know is whether that's because my port is too big as per BBs suggestion, i.e. if I reduced the port maybe I wouldn't need to lighten the piston so much ?

This short port gun is my project for next week, but it really is shooting lovely right now, and pretty efficient - I reckon I could shave another 5mm off the stroke and still make 11 easy, as it was doing bang on 12 before I chopped the spring; that took it down to 11.5, then it crept up to 11.9 again before I collapsed the coil to get back to 11.6. So the current powerplant at 75mm is definitely capable of 12.5 - and that's smoothly, no slamming and somewhat firm but nothing silly on cocking effort.

Will update early next week when I open her up, right now I'm in the tedious process of grinding dodgy comp tubes so they actually seal As the Doc says, bring on those CNC drop in short port replacement tubes (no point just doing the end plug IMHO, the tubes are such a pain )

I'm still trying to work out the the 5.3mm long port offers much advantage over the 7.5 - but it does deliver more power for the stroke... maybe when I sleave the port down as per BBs model it'll get better, but right now they shoot "different", but if anything the 7.5mm port gun feels slightly "calmer"...

HW piston heads sound good, nice easy way to do it, put me down for a tester - hopefully I'll have my "experimental" gun up and running by then, save keeping tearing them apart

Cheers - JB

[lutonlad]

I wonder if someone could eloquently try to explain the feeling or sensations of piston bounce, slam, twisting , etc.. alongside the physics of what is happening inside the gun?

I'd also like to know how people sense piston bounce, and piston slam.

[Olas]

I'd also like to know how people sense piston bounce, and piston slam.

X2!!

A well articulated explanation of the tactile sensation associated with these phenomena would be marvelous for a spring-amateur such as myself..

[Shed tuner]

Hi BB,

tried the 75mm, 5.3 x 3.8mm port gun with a 3.2 port instead.

3.8 port 8.4s: 768, 7.9s: 796 (so slight bounce)
3.2 port 8.4s: 722, 7.9s: 760 (lots more bounce and lots less power)

I then removed port and reassembled and it came back (withing a couple of fps) to the original/ Consistency was +-5fps throughout.

Empirically this is as expected, as when I see bounce, reducing the port can only reduce power/increase bounce. For shrinking the port to help, I'd have had to have seen slam - all in my experience anyways....

By way of more data I then added a 2mm preload spacer (wanting to get rid of the little bounce, plus a little spring guide grease ) and got:
790/813 (bit close - shows how easy this thing can make power though - looks like that washer needs to come back out)

Dave, preload is around 40mm, wire is 3.2, OD 20.6, forgot to count coils but it's a pretty "average" pitch around 8mm. Will measure it when I remove that washer !

Rgds - JB

[Shed tuner]

bump for Mr Beard (who's inbox is overflowing)

[Black Beard]

Interesting. I can replicate that if I change the entrance/exit parameters of the TP to increase the drag. The TP would have to have less smooth changes in shape than I'd like- like sudden step entrances/exits. But here are so many assumed vaues that it could still be completely wrong.

Thanks for that. All useful in understanding how it all works.

BB

[tokoloshe]

Interesting. I can replicate that if I change the entrance/exit parameters of the TP to increase the drag. The TP would have to have less smooth changes in shape than I'd like- like sudden step entrances/exits. But here are so many assumed vaues that it could still be completely wrong.

Thanks for that. All useful in understanding how it all works.

BB

BB, I got some similar results to Jon when testing TP sizes. I started at 3mm and worked my way up to 3.9 in 1mm increments. Each time I drilled out I gave the entrance a tickle with a center drill so I had a beveled entrance and not just a step. Have done this excersize twice now, but using a different power plant on 2nd go.
From memory power was down ~ 30fps, maybe a little more at start, but stayed consistant until around 3.5mm reached. Cycle wasn't bouncing much, though experienced more of a harder feel/stop at end of cycle. Things change quickly from 3.5 to 3.7. Power started to increase and shot cycle feel improved. No real gains seen over 3.7 other than more bounce at 3.9 which is contradicting JB! I have these notes somewhere, will dig them out to confirm memory! When using an O ring in my TX I find the 3.5 TP about right to control bounce, will go to 3.7 if using para seals. Indications are AA got TP size right!

[Shed tuner]

Yeah, port was slightly radiused at entrance each time, but just tidied at exit to keep a "straight flow", into the barrel, ie not introduce dispersion...
All the Hw77/97 boys, with their ~6mm long ports, seem to have settled on ~3.8/3.9 or even 4.0, so stock 3.8 does seem a good place to start in the shorter port TXs (this 5.3, and the 7.5s). For long port TXs (10mm), I'm guessing 3.4 to 3.6 might work better - will try to get time to try it.

[brucegill]

I've got some figures here that may (or not) help....

7.5mm x 3.8mm port
80mm stroke
236g piston
36mm preload
Wonkey Donkey spring

8.4g - 779fps (11.3ftlb)
7.9g - 803fps (11.34)
7.3g - 824fps (11.02)

same as above but now 223g piston (inc. rod, seal, bearings but not the delrin top hat). Now 35mm preload

8.4g - 779fps (11.34)
7.9g - 808fps (11.45)
7.39 - 831fps (11.2)

after reading some of the above I May radius the inner edge on the TP as it just had a de-burr. I took 2.5mm off the inside face of the comp tube end.

I'd only taken 13g off the piston as I wanted to go slowly. Looking at the figures I'm not sure I'll gain much taking any more off?

Comp tube was honed (brass bar and grinding paste, then autosol) and polished. This defo helped consistency and became more efficient.

Stroke was at 78mm but the ABT was a pain in the arse so went to 80 and now it clicks the first notch and then the sears engage. (Mk3 rifle)

[Shed tuner]

nicely done bruce, those last figures look perfect - leave it there and enjoy...

Tell me I was right about you liking the short port conversion ? :-)

[brucegill]

nicely done bruce, those last figures look perfect - leave it there and enjoy...

Tell me I was right about you liking the short port conversion ? :-)

Thanks Jon Well chuffed! loving the short port shot cycle! Many thanks.

shot around 150-200 through it and felt like the power had dropped . Just nipped up to the workshop and a quick check on the skan:

8.4g - 781 (11.38)
7.9 - 803 (11.3)
7.3 - 833 (11.25)

seems odd that the numbers have changed in proportion to each other after settling down. . Will check again after a tin or two.

haven't got long until the worlds so think im going to leave this as is and get some practise in lol

this weight loss changed the consistency of these jsb's as well. Was 6fps spread for 8.4, now 10. Was 3fps for the 7.9, now 5. Was 7fps for the 7.3, now 3. Again, will check after a few tins.

currently restoring a very early mk1 .177 so will change spring and guides to the same as the mk3 above but keep stock 83mm stroke and long TP. then I can really compare the two setups.